Just to give you some practice I have found this DOF simulator. It will help you get your head around DOF, focal length and so on. For those of you viewing this from a mobile phone it is much better on a PC or laptop because when you change the settings you can see what happens to the image.
To change the focal length go the lens bar and move the slider on the camera icon. It start at 24mm which is a wide angled lens and goes all the way up 300mm which is a telephoto lens.